1. Field of the Invention
The invention relates to a substrate support chuck for electrostatically retaining a workpiece upon the support surface of the chuck. More particularly, the invention relates to a bipolar electrostatic chuck having apparatus for balancing the electrostatic force applied to a workpiece supported by the chuck.
2. Description of the Background Art
Substrate support chucks are widely used to support substrates within semiconductor processing systems. One example of an electrostatic chuck is described in commonly assigned European Patent Publication Number 0 439 000 B1, published Sep. 14, 1994. This electrostatic chuck has a conventional chuck body containing a dielectric material having a pair of coplanar electrodes embedded therein. The electrodes are half-moon or D-shaped such that each electrode provides clamping force to half the workpiece that is supported by a support surface of the chuck body.
In operation, a chucking voltage is applied to each electrode which causes an electric field to form between the electrodes. This electric field causes charge to distribute on the underside of the wafer that is oppositely polarized to charge located on the chuck surface. The Coulomb force between the charge on the wafer and the charge on the chuck surface attracts the wafer to the chuck. As such, the wafer is retained (clamped) upon the chuck surface.
Ideally, the electrostatic force that retains the wafer should be uniform across the entire underside of the wafer. However, in reality, this electrostatic force may vary substantially across the wafer. The force may vary because of differences in dielectric thickness, wafer backside roughness, chuck surface roughness, variations in chuck surface conductivity and the like. These differences cause an uneven distribution of clamping voltage relative to each electrode.
For example, in a bipolar, dielectric electrostatic chuck such as that described in European Patent discussed above, the combination of the two electrodes and the wafer form, in effect, a pair of series connected capacitors. If, for example, the spacing between the wafer and each of the electrodes is unequal, the voltage drop from one electrode to the wafer (first capacitor) will be different from the voltage drop across the space between the other electrode and the wafer (second capacitor). These unequal voltage drops will cause unequal clamping forces to be applied to each half of the wafer.
As another example, in a bipolar, ceramic electrostatic chuck (e.g., a Johnsen-Rahbek type chuck such as is disclosed in U.S. Pat. No. 5,117,121 issued May 26, 1992), each electrode and the wafer form, in effect, a pair of series connected resistors. If, for example, the spacing between the wafer and each of the electrodes is unequal, the voltage drop from one electrode to the wafer (first resistor) will be different from the voltage drop across the space between the other electrode and the wafer (second resistor). As a result of these unequal voltage drops, unequal clamping forces are applied to each half of the wafer.
One example of apparatus used to balance these electrostatic forces is disclosed in the European Patent cited above. This patent discloses a bipolar dielectric chuck that is driven by a dual power supply. The center tap of the dual power supply is connected to a reference electrode that extends into a plasma that is formed proximate the wafer during wafer processing. As such, the center tap of the power supply attains the plasma voltage, i.e., the power supply voltages that are applied to the electrodes are referenced to the plasma potential. Thus, the wafer, which is generally in contact with the plasma, is indirectly connected to the reference contact. Consequently, an unequal voltage drop from the wafer to each of the electrodes would be adjusted to achieve equal voltages as the center tap voltage changed to compensate for the unequal voltage drops.
However, to achieve this balancing effect, a plasma must be generated proximate the wafer and the reference contact exposed to the plasma. In some semiconductor processing applications, such as physical vapor deposition (PVD) processes, the wafer is processed, e.g., heated, prior to being exposed to a plasma. Thus, the apparatus of the above described chuck that requires the presence of a plasma would not be effective until processing began. As such, the foregoing chuck arrangement is not useful for processes that do not utilize plasmas or do not utilize plasmas for the entire process period.
Therefore, a need exists in the art for apparatus and a concomitant method of automatically balancing the electrostatic force between an electrostatic chuck and a workpiece without relying on the presence of a plasma proximate the workpiece.